1. Field of the Invention
The invention relates to a method of determining a nuclear magnetization distribution in a region of a body which is situated in a generated steady, uniform magnetic field, which method includes measurements comprising the following steps:
(a) generating an RF pulse for causing a precessional motion of a local magnetization in the region, a resonance signal thus being generated;
(b) sampling, during a measurement period, the resonance signal influenced by at least one magnetic field gradient;
(c) repeating n times, where n is an integer, measurement cycles including the steps (a) and (b), possibly with a varying value of the time integral of the magnetic field gradients specified sub (b) and/or with a varying direction of the magnetic field gradients specified sub (b), said method eliminating phase errors in pixels of a complex image of the nuclear magnetization distribution, after which corrected phases have the values 0 or .pi..
The invention also relates to a device for determining a nuclear magnetization distribution in a region of a body, which device comprises:
(a) means for generating a steady, uniform magnetic field;
(b) means for generating an RF pulse;
(c) means for generating a magnetic field gradient;
(d) sampling means for sampling, during a measurement period, a resonance signal generated by means of the means specified sub (a) and (b) and influenced by at least one magnetic field gradient;
(e) processing means for processing signals supplied by the sampling means, and
(f) control means for controlling the means specified sub (b) to (e) for generating, sampling and processing a number of resonance signals, the control means supplying control signals to the means specified sub (c) for the possible adjustment of the strength or duration and/or direction of the magnetic field gradient, the integral of the strength over the duration and/or direction of the magnetic field gradient possibly being different after each repetition of cycles.
Herein, the term nuclear magnetization distribution is to be understood to cover a spin density distribution, a flow velocity distribution, a relaxation time T.sub.1, T.sub.2 distribution, as well as a spin resonance frequency spectrum distribution (N.M.R. location-dependent spectroscopy), etc.
2. Description of the Prior Art
A method of this kind is known from "Book of Abstracts" of the Fourth Annual Meeting, Society of Magnetic Resonance in Medicine, London 1985, page 495, which contains an abstract by P. Margosian of a poster session presented on Aug. 22, 1985, in London.
Devices for determining a nuclear magnetization distribution in a region of a body and the principles on which such devices are based are known, for example from the article by Locher "Proton NMR Tomography", Philips Technical Review, Vol. 41, 1983/84, No. 3, pages 73-78. Reference is made to the cited article for the description of their construction and operating principles. The description of the apparatus, pulse sequences and image reconstruction method in the article by Locher are incorporated herein by way of reference.
A method described in the abstract by P. Margosian involves a so-called conventional spin echo method. Using such a method, a body to be examined is subjected to a strong, steady, uniform magnetic field Bo whose direction coincides with, for example the z-axis of a cartesian coordinate system (x, y, z). The steady magnetic field Bo realizes a slight polarization of the spin nuclei present in the body and enables spin nuclei to perform a precessional motion about the direction of the magnetic field Bo. After application of the magnetic field Bo there is applied a magnetic field gradient which acts as a selection gradient; at the same time a 90.degree. RF pulse is generated which rotates the magnetization direction of the spin nuclei present in a selected slice through an angle of 90.degree.. After termination of the 90.degree. pulse the spin nuclei will perform a precessional motion about the field direction of the magnetic field Bo, thus generating a resonance signal (FID signal). After the 90.degree. pulse, field gradients G.sub.y, G.sub.x and G.sub.z are simultaneously applied, the field direction thereof coinciding with that of the magnetic field Bo, their gradient directions extending in the y-direction, the x-direction and the z-direction, respectively. The field gradients G.sub.x, G.sub.y and G.sub.z serve for rephasing and encoding the spin nuclei in the x-direction, the y-direction and the z-direction, respectively. After termination of the three field gradients a field gradient G.sub.x is applied, after a 180.degree. echo pulse, an echo resonance signal of the original FID signal then being sampled.
In order to obtain an image of the selected region, a measurement cycle is repeated a number of times, each time using a different value of the time integral of the field gradient G.sub.y and/or G.sub.z in each cycle. By arranging the Fourier transforms of the resonance signals in an ascending order of magnitude of the time integral of the field gradient G.sub.y on the one hand and of the field gradient G.sub.z on the other hand and by subjecting these transforms to a Fourier transformation in the y-direction and subsequently in the z-direction, for example a spin density distribution is obtained as a function of x, y and z.
When the excited magnetizations in the selected region of the body concern, for example protons in water as well as fat, the magnetizations of water protons as well as fat protons will perform a precessional motion about the direction of the magnetic field Bo under the influence of the RF 90.degree. pulse. Because the precessional motion of the magnetizations of the water protons is approximately 3.5 ppm faster than that of the magnetizations of the fat protons, said difference in precessional frequency amounting to approximately 70 Hz in the case of a steady uniform magnetic field of 0.50 T, almost immediately after the RF 90.degree. pulse the direction of the magnetizations of the water protons will no longer be the same as that of the fat protons. Any complex image of the selected region thus obtained will then constitute a so-called combined water/fat image. Inter alia from "Simple Proton Spectroscopic Imaging", W. Th. Dixon, Radiology 153 (1984), pages 189-194, it is known that, when separate D water images and fat images of the selected region are desired, it is necessary to form two images, each image corresponding to a respective situation in the selected region in which the magnetization of the water protons are directed in the same direction (and are positive real) and the opposite direction (and are negative real), respectively, with respect to those of the fat protons (which are positive real). By subtraction and addition of these two images, a separate water image and fat image, respectively, can be formed. However, in a measurement cycle in practice it will be difficult to create the situation in which the water protons and the fat protons have the same phase or exhibit a phase difference of .pi. rad. This is because the phases of the respective protons are also influenced by, for example, inhomogeneities of the steady, uniform magnetic field, instabilities of the magnetic field gradients, and eddy currents, so that the pixel values will no longer be purely real and the phases will contain a phase error component which differs from one pixel to another.
In the cited abstract by P. Margosian a method is proposed for estimating phase error contributions by notably inhomogeneities of the steady uniform magnetic field. P. Margosian proposes to utilize a phantom object filled only with water in order to determine such phase error contributions by magnetic field inhomogeneities. When the phase error contributions per pixel are known, a phase correction can also be performed per pixel; corrected phases then have the values 0 or .pi. (corresponding to water or fat). First of all, a previously described conventional spin echo measurement is performed, the RF 180.degree. pulse being generated at such an instant that the effects of the homogeneities of the steady uniform magnetic field at the centre of the magnetic (measurement) field gradient generated after the 180.degree. pulse are eliminated. This is possible when the RF 180.degree. pulse is symmetrically situated with respect to the RF 90.degree. (excitation) pulse and the centre of this magnetic measurement field gradient. This is because the phase error contributions due to the inhomogeneities of the steady uniform magnetic field prior to and subsequent to the instant of generation of the 180.degree. pulse then cancel one another at the centre of the magnetic measurement field gradient. Subsequently, there is performed a second conventional spin echo measurement which is identical to the first measurement, except that the RF 180.degree. pulse is now shifted with respect to that during the first measurement. When the respective images of the first and the second measurement are compared one pixel after the other, the phase error contributions by the (magnetic field) inhomogeneities of the steady uniform magnetic field can be determined for each pixel, because they occur only in the image of the second measurement.
It is a drawback of the known method that no correction is made for phase error contributions by inhomogeneities of the steady uniform magnetic field which are caused by the magnetic susceptibility of, for example a patient to be examined. Moreover, the inhomogeneities of the above magnetic field which are caused by eddy currents are not properly corrected, because the phase error contributions by eddy currents vary in different repetitions of a measurement cycle due to a difference in strength of the gradient (gradients). It is also a drawback that information as regards the inhomogeneities of the steady uniform magnetic field in all regions which can possibly be selected must be known before it is known which region of a body is to be selected.